1. Field of the Invention
The present invention relates to a tool clamping device used for a machine tool.
2. Description of the Related Art
A tool clamping device used for a machine tool, particularly a machining center, needs to generate a predetermined force for clamping a tool. However, the generation of such a force only by a spring may increase the size of the tool clamping device. In order to avoid a size increase of the tool clamping device, the tool clamping device includes a mechanism for magnifying a spring force.
FIG. 1 is an overall schematic diagram of the tool clamping device. A main shaft 1 contains a draw bar 3 inserted via a spring 2. Reference numeral 6 denotes a tool holder.
FIG. 5 is an enlarged view showing a part A of FIG. 1. A concave portion 9 is formed on the end of the draw bar 3 while a pull stud 7 is fit into the concave portion 9. Furthermore, a guide hole 10 is provided on a cylindrical part 4 of the draw bar 3. Steel balls 5 are fit into the guide hole 10. Moreover, an inclined surface 12 is provided on the pull stud 7 in contact with the steel balls 5 while an inclined surface 11 is provided on the inner surface of the main shaft 1 so as to be substantially opposed to the inclined surface 12 provided on the pull stud 7 with the steel balls 5 interposed between the inclined surfaces 11 and 12. Reference numeral 8 denotes a main-shaft inner wall surface.
With this configuration, when the draw bar 3 and the pull stud 7 are lifted by a force of the spring 2, a reaction force is generated by the spring 2 between the steel ball 5 and the inclined surface 12 of the pull stud 7 and between the steel ball 5 and the inclined surface 11 of the main shaft 1, whereupon a magnification effect that magnifies the force of the spring 2 is generated.
Japanese Utility Model Application Laid-Open No. 2-104906 discloses a tool holding device that includes a tapered sleeve on the opposite side of a lead-in bar provided with a tool clamping mechanism. The tool holding device includes a magnification mechanism composed of the tapered sleeve, steel balls, and an inclined surface provided on a main shaft.
Japanese Patent Application Laid-Open No. 1-205905 discloses a tool clamping mechanism in which a magnification mechanism has an inclined surface on a pull stud provided in a tool holder, steel balls on a draw bar, and an inclined surface provided on a main shaft.
In any of prior art techniques illustrated in FIG. 5 and disclosed in Japanese Utility Model Application Laid-Open No. 2-104906 and Japanese Patent Application Laid-Open No. 1-205905 described above, the magnification mechanism has the inclined surface on the inner wall surface of the main shaft. The inclined surface is formed by cutting or the like during the manufacturing of the main shaft. Thus, the manufacturing of the tool clamping device requires working for forming the inclined surface on the inner wall surface of the main shaft. This may increase the number of man-hours required for working, increasing the manufacturing cost of the tool clamping device.